Let AppTransition has its own Handler.

The CL is for the preparation of per display AppTransition refectoring.

It includes:
- Add Handler for processing AppTransition related messages.
- Moving APP_TRANSITION_TIMEOUT & DO_ANIMATION_CALLBACK logics from
  WMS to AppTransition class.

Bug: 111362605
Test: go/wm-smoke
Test: atest ActivityManagerTransitionSelectionTests
Test: atest WindowSurfacePlacerTest AppTransitionTests
Test: atest BoundsAnimationControllerTests
Change-Id: Ifb6bcac794d8212d41eb6018526bd4633c19138d